1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a heavy duty pneumatic radial tire for use in relatively large-size vehicles such as truck, bus and the like and improving a resistance to uneven wear in a tread rubber.
2. Description of Related Art
In a tread portion of the pneumatic tire, a crown part forming a ground contact area usually has a convex shape made of a plurality of arcs smoothly connected to each other at a section of the tire for adequately holding a ground contact form or a ground contact pressure distribution. Such a convex shape of the crown part can avoid a case that when the tire is run under loading, a ground contact pressure in a shoulder region of the tread portion directly subjected to an influence of a stiffness in a sidewall portion becomes higher than that of the other region.
On the other hand, a diameter of a central region in the crown part is made larger than a diameter of the shoulder region by rendering the crown part into the convex shape, so that the following force is created in the ground contact face of the tread portion. That is, since a peripheral length of the central region is longer than a peripheral length of the shoulder region, when the tire is run under loading, a forward force directing to a running direction of the tire is created in the ground contact face of the central region, while a rearward force directing to a braking direction of the tire is created in a ground contact face of the shoulder region.
Under the action of these forward and rearward forces, a worn amount of a tread rubber in the central region strongly subjected to an influence of a traction force increases in a tire mounted onto a driving shaft, while a worn amount of a tread rubber in the shoulder region strongly subjected to an influence of a braking force increases in a tire mounted onto a non-driving shaft. Since the increase of these rubber worn amounts is compared with a rubber worn amount of the other region, uneven wear is caused in the tread rubber of the crown part. Particularly, such a remarkable wear in the central region of the tire mounted onto the driving shaft is so-called center wear and is a serious problem. However, it is impossible to largely change the convex shape of the crown part because of the above reasons.
It is, therefore, an object of the invention to provide a pneumatic tire having an excellent resistance to uneven wear by largely mitigating the forward and rearward forces generated while holding the convex shape of the crown part to prevent the occurrence of uneven wear of a tread rubber, particularly center wear.
According to the invention, there is the provision of a pneumatic tire comprising a pair of bead portions, a pair of sidewall portions, a tread portion, a radial carcass of at least one ply reinforcing these portions, and a belt reinforcing the tread portion; said tread portion being provided with two or more circumferential main grooves formed in a tread rubber at an inner region sandwiched by both shoulder regions and one or more circumferential ribs defined by two adjoining circumferential main grooves, wherein at least two circumferential fine grooves are formed in the circumferential rib.
In this case, definitions of the tire are decided by industrial standards effective in zones manufacturing or using tires. For example, they are defined by xe2x80x9cYear Books of The Tire and Rim Association Inc.xe2x80x9d in USA, xe2x80x9cStandards Manual of The European Tire and Rim Technical Organizationxe2x80x9d in Europe, or xe2x80x9cJATMA Year Book of The Japan Automobile Tire Manufactures Association, Inc.xe2x80x9d in Japan.
In a preferable embodiment of the invention, each of the circumferential fine grooves in the circumferential rib has such a form that a bisector putting at a center of a groove width of the fine groove in a direction of a groove depth thereof is parallel to a normal line passing through a center of a width of a rib portion defined by the two adjoining circumferential fine grooves.
In another preferable embodiment of the invention, each of the circumferential fine grooves in the circumferential rib has such a form that a bisector putting at a center of a groove width of the fine groove in a direction of a groove depth thereof is inclined with respect to a normal line passing through a center of a width of a rib portion defined by the two adjoining circumferential fine grooves so as to widen toward an inside of the tire.
In the other preferable embodiment of the invention, a surface of a rib portion defined by the two adjoining circumferential fine grooves is offset radially inward with respect to a surface of a rib portion defined by the circumferential groove and the circumferential fine groove adjacent thereto.
In a still further preferable embodiment of the invention, the two adjoining circumferential fine grooves are extended zigzag in the circumferential direction of the tread portion so as to form constricted parts corresponding to knots and width-enlarged parts corresponding to bellies in the rib portion defined by these circumferential fine grooves. In this case, a difference between a maximum width of the width-enlarged part and a minimum width of the constricted part as to the width in a rotating axial direction of the tire is larger in a surface position of the rib portion than in a bottom position of the circumferential fine groove. In a further preferable embodiment of the invention, the two adjoining circumferential fine grooves have a straight bottom shape.